A conventional vehicle automatic transmission has a torque converter and a multiple stage transmission gear mechanism connected with the torque converter. The gear mechanism includes hydraulically applied friction members for selecting one of the gear ratios of the transmission gear mechanism in accordance with vehicle operating conditions. A hydraulic pump produces pressure required to operate the friction members and control valves.
The friction members are controlled by a hydraulic control system. The hydraulic control system comprises pressure regulating means for regulating hydraulic pressure generated from the hydraulic pump, hydraulic control means for controlling shift shock and shift-responsiveness, damper clutch control means for controlling a damper clutch of the torque converter, and hydraulic pressure supplying means for supplying hydraulic pressure to each friction member.
The hydraulic control means controls supply line pressure acting on each friction member, torque converter supply pressure, solenoid valve supply pressure. These pressures substantially affect the smoothness of shifting.
Therefore, there is provided a conventional hydraulic control system which initially feeds torque pressure to the friction members and then replaces the torque pressure with drive pressure when synchronization is completed.
In such a hydraulic control system, each valve is provided with a solenoid valve which is duty-controlled so as to operate the damper clutch, vary line pressure, and supply torque pressure when initially shifting.
However, during the duty control of the solenoid valves for controlling the shift operation, fluid is drained through the solenoid valves, such that a stable level of pressure cannot be maintained. That is, it is difficult to stabilize the hydraulic pressure due to fluid loss, resulting in shift shock.